N-(phosphonomethyl)glycine (also known in the agricultural chemical industry as glyphosate) and its salts are conveniently applied as a component of aqueous, post-emergent herbicide formulations. As such, they are particularly useful as a highly effective and commercially important broad-spectrum herbicide for killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation and aquatic plants.
One of the more widely accepted methods of making N-(phosphonomethyl)glycine comprises the liquid phase oxidative cleavage of a carboxymethyl substituent from an N-(phosphonomethyl)iminodiacetic acid substrate using an oxygen-containing gas in the presence of a heterogeneous oxidation catalyst. As used herein, “N-(phosphonomethyl)iminodiacetic acid substrates” include N-(phosphonomethyl)iminodiacetic acid and salts thereof, wherein the salt-forming cation is, for example, ammonium, alkylammonium, an alkali metal or an alkaline earth metal. N-(phosphonomethyl)glycine may be prepared by the liquid phase oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid with oxygen in accordance with the following reaction: Other by-products also may form, such as formic acid, which is formed by oxidation of the formaldehyde by-product and aminomethylphosphonic acid (AMPA), which is formed by oxidation of N-(phosphonomethyl)glycine. The preference for heterogenous catalysis stems, at least in part, from the relative ease with which a particulate heterogeneous catalyst can normally be separated from the reaction product mixture for reuse following the oxidation. The literature is replete with examples of heterogeneous catalysts. See generally, Franz, et al., Glyphosate: A Unique Global Herbicide (ACS Monograph 189, 1997) at pp. 233-62 (and references cited therein); Franz, U.S. Pat. No. 3,950,402; Hershman, U.S. Pat. No. 3,969,398; Felthouse, U.S. Pat. No. 4,582,650; Chou, U.S. Pat. Nos. 4,624,937 and 4,696,772; Ramon et al., U.S. Pat. No. 5,179,228; Ebner et al., U.S. Pat. No. 6,417,133; and Leiber et al., U.S. Pat. No. 6,586,621.
A high concentration of formaldehyde in the reaction product mixture resulting from the oxidative cleavage of an N-(phosphonomethyl)iminodiacetic acid substrate is undesirable. In particular, the formaldehyde by-product is undesirable because it tends to react with the N-(phosphonomethyl)glycine product to produce further unwanted by-products including N-methyl-N-(phosphonomethyl)glycine (NMG), which reduces N-(phosphonomethyl)glycine yield. In addition, the formaldehyde by-product itself is undesirable because of its potential toxicity. See Smith, U.S. Pat. No. 5,606,107.
Franz, U.S. Pat. No. 3,950,402, discloses oxidizing the formaldehyde by-product to carbon dioxide and water simultaneously with the oxidative cleavage of the N-(phosphonomethyl)iminodiacetic acid substrate by using a heterogenous oxidation catalyst comprising a noble metal deposited on a carbon support. Such noble metal on carbon oxidation catalysts are referred to as “bifunctional” as the carbon component provides the primary adsorption site for the oxidation of the N-(phosphonomethyl)iminodiacetic acid substrate to form the N-(phosphonomethyl)glycine product and formaldehyde, while the noble metal component provides the primary adsorption site for the oxidation of formaldehyde and formic acid to form carbon dioxide and water. The noble metal component may also tend to reduce the rate of deactivation of the catalyst (i.e., prolong the useful life of the catalyst). The overall reaction is summarized as follows: However, under typical conditions of the oxidation reaction, some of the noble metal in the catalyst used by Franz is oxidized into a more soluble form and both the N-(phosphonomethyl)iminodiacetic acid and N-(phosphonomethyl)glycine product act as chelating ligands that tend to solubilize the noble metal. Thus, even though the process disclosed by Franz produces an acceptable yield and purity of N-(phosphonomethyl)glycine, high losses of the costly noble metal by dissolution into the aqueous reaction solution (i.e., leaching) undermine the economic feasibility of the process.
Ramon et al., U.S. Pat. No. 5,179,228, disclose a process for the preparation of N-(phosphonomethyl)glycine by oxidation of N-(phosphonomethyl)iminodiacetic acid using an oxygen-containing gas in the presence of a noble metal on an activated carbon catalyst. Recognizing the problem of leaching attendant the use of a noble metal on carbon catalyst in the oxidation of an N-(phosphonomethyl)iminodiacetic acid substrate (noble metal losses as great as 30% are reported), Ramon et al. propose flushing the reaction mixture with nitrogen gas under pressure after the oxidation reaction is complete. According to Ramon et al., nitrogen flushing causes redeposition of solubilized noble metal onto the surface of the carbon support and reduces the noble metal loss to less than 1%. However, the amount of noble metal loss incurred with this method is still unacceptable. In addition, re-depositing the noble metal can lead to a loss of noble metal surface area which, in turn, decreases the activity of the catalyst.
Using a different approach, Felthouse, U.S. Pat. No. 4,582,650, discloses using two catalysts: (i) an activated carbon to effect the oxidation of N-(phosphonomethyl)iminodiacetic acid into N-(phosphonomethyl)glycine; and (ii) a co-catalyst to concurrently effect the oxidation of formaldehyde into carbon dioxide and water. The co-catalyst consists of an aluminosilicate support having a noble metal located within its pores. The pores are sized to exclude N-(phosphonomethyl)glycine and thereby prevent the noble metal of the co-catalyst from being poisoned by N-(phosphonomethyl)glycine. According to Felthouse, use of these two catalysts together allows for the simultaneous oxidation of N-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine and of formaldehyde to carbon dioxide and water. This approach, however, suffers from several disadvantages: (1) it is difficult to recover the costly noble metal from the aluminosilicate support for re-use; (2) it is difficult to design the two catalysts so that the rates between them are matched; and (3) the carbon support, which has no noble metal deposited on its surface, tends to deactivate at a rate which can exceed 10% per cycle.
More recently, attention has focused on developing bifunctional noble metal on carbon oxidation catalysts which resist noble metal leaching (i.e., exhibit improved compositional stability) and provide increased activity and/or selectivity, particularly with respect to oxidation of formaldehyde into carbon dioxide and water (i.e., increased formaldehyde activity). Ebner et al., U.S. Pat. No. 6,417,133, disclose “deeply reduced” noble metal on carbon catalysts containing various metal promoters for use in the oxidative cleavage of an N-(phosphonomethyl)iminodiacetic acid substrate and oxidation of other oxidizable reagents and methods for their preparation. Such deeply reduced catalysts exhibit remarkable resistance to noble metal leaching in aqueous, acidic, oxidation reaction media. As a result, the catalyst disclosed by Ebner et al. provides for substantially quantitative oxidation of N-(phosphonomethyl)iminodiacetic acid substrates to N-(phosphonomethyl)glycine products, while maintaining effective oxidation of the formaldehyde and formic acid by-products of the reaction for a prolonged period and/or over numerous oxidation cycles. Still, the process of Ebner et al. typically does not eliminate all the formaldehyde and formic acid by-product and, consequently, also does not eliminate all the NMG. Accordingly, a need persists for improvements which might further reduce noble metal losses, provide increased catalyst stability, activity and/or selectivity, particularly in the oxidation of formaldehyde and other N-(phosphonomethyl)iminodiacetic acid substrate oxidation by-products, and generally extend the useful life of such catalysts.
Tellurium has been described as a promoter metal for use in liquid phase oxidation reactions. For example, in WO 00/01707, Siebenhaar et al. describe oxidizing salts of N-(phosphonomethyl)iminodiacetic acid in the presence of a noble metal on carbon catalyst containing 0.5% to 10% of a doping metal based on the weight of the carbon support. Although the disclosure includes tellurium in a list of potential doping metals, the principal teaching of the reference is directed to the use of commercially prepared noble metal on carbon catalysts doped with bismuth or lead.